The two rows of 20 characters have no space row between them, so that using the 8 programmable characters you can create characters that span both rows. The Canon MX300 printer displays a mix of single and double height digits, but I have no information on whether the controller supports this or whether they just used programmable characters. During testing I tried setting double height fonts but to no useful effect. See mx300_driver_library for drivers that implement double height characters.